Heretofore, it has frequently been necessary to repair various types of pipe and tube by cutting a section of the pipe out of the line and inserting a new section. Normally, the new section would be welded to the cut ends of the adjacent pipe lengths. In accordance with preferred welding practice, a groove should be provided at the point of weld to permit the molten metal to weld to a relatively large surface area of the pipes. In order to do this, the adjoining ends of the pipe lengths to be welded should be provided with a frustoconical bevel so that when the two ends are placed in alignment, a circumferential groove is formed to receive the molten metal from the welding rod.
Accordingly, the preferred practice is to mill a frustoconical bevel on the respective ends of the pipe lengths.
In many instances, such as in replacing sections of corroded pipe in a boiler, it is necessary to mill the cut end of a pipe which cannot be removed from its installed position. Accordingly, it is desirable to have a portable milling tool which the operator can carry to the desired location and accurately position in alignment with the pipe. Also, it is desirable that the tool be an air pressure-operated type tool, since many working environments might have volatile gases that could be ignited by electric arcing from an electrical power tool.
The prior art devices used for milling operations of this type have utilized an air pressure-operated motor arranged with it the axis of rotation at right angles to the axis of rotation of the milling cutter. Power is transferred from the air motor to the milling cutter through a power transfer arrangement including bevel gears, such as is shown in prior art U.S. Pat. Nos. 4,620,823 and 4,761,104. The power transfer system shown in those patents provides a substantial gear reduction so that the milling head rotates no faster than at a speed of around 100 RPM. These slow speeds require an axial guide pin that is inserted and secured in the end of the tube or pipe to stabilize the milling head during the cutting operation.
More importantly, these prior art devices with a gear reduction arrangement cause great stresses on the gear teeth and, as a result, the teeth are often damaged after only a short period of operation, requiring rebuilding of the tool.
Another problem that is encountered with prior art devices is stalling of the air motor during the grinding operation. Standard air motors used in tools of this type generally develop a high torque at a relatively low speed. Most standard air motors have a relatively large lobe to provide a high chamber expansion in a short axial length. This design provides a relatively large polar moment of inertia for the rotor and thus a relatively slow speed. The result is a high torque and a tendency to stall when too much resistance is encountered.
The tool of the present invention resolves the difficulties indicated above, and affords other features and advantages heretofore not obtainable.